In this specification we will refer to airborne surveys, and more particularly to gravity surveys. However the techniques we describe are not limited to these types of survey and may be applied to other potential field surveys including, but not limited to, magnetic field surveys such as magnetotelluric surveys, electromagnetic surveys and the like.
A potential field survey is performed by measuring potential field data which, for a gravity survey, may comprise one or more of gravimeter data (measuring gravity field) or gravity gradiometer data (measuring gravity field gradient), vector magnetometer data, true magnetic gradiometer data, and other types of data well-known to those skilled in the art. A common aim of a geophysical potential field survey is to search for signatures which potentially indicate valuable mineral deposits. Conventionally airborne potential field surveys such as gravity surveys are flown on a grid pattern. The grid is defined by orthogonal sets of parallel lines (flight paths) on a two-dimensional surface which is draped over the underlying terrain. However the draped surface is constrained by the closest the aircraft is permitted to fly to the ground and the maximum rate of climb/descent of the aircraft. Some improved techniques for airborne potential field surveys, which facilitate the collection of data from close to the ground, are described in the applicant's co-pending PCT patent application “Gravity Survey Data Processing”, PCT/GB2006/050211, hereby incorporated by reference in its entirety.
After the potential field data has been collected but prior to interpreting the data a terrain correction is generally applied, compensating for surface height variations. A surface profile may be purchased in the form of digital terrain elevation data or determined from (D)GPS ((Differential) Global Position System) and/or airborne techniques such as LIDAR (Laser Imaging Detection and Ranging) and SAR (synthetic aperture radar). Aircraft acceleration, attitude, angular rate and angular acceleration data may also be used to correct the output data of the potential field instrumentation. We describe some improved techniques for terrain correction in geophysical surveys in our co-pending UK patent application “Terrain Correction Systems”, no. 0601482.3, filed 25 Jan. 2006, also hereby incorporated by reference in its entirety.
However despite the improved techniques we have previously described low frequency noise or drift in airborne survey measurements remains a problem. (Although the term drift sometimes refers to a random monotonic variation, in this specification it is used to represent any form of correlated noise with a characteristic frequency much less than dominant signals of interest).
The term “levelling” is used in the art as a generic term to cover conventional techniques for data conditioning. These techniques include removal of low frequency drift, matching low frequency content of neighbouring lines, and referencing data to a fixed height plane. For example the intersection points of a standard gridded survey can be used for cross-over levelling, where the data along survey lines are adjusted to minimise differences at these points.
There is, however, a need for improved data processing and, in particular, for improved handling of noise.